This invention relates to a method for the dry cleaning of grain by means of screens and gravity separation, more particularly for the removal of undesired impurities, e.g. parts of high specific gravity, such as sand, and lightweight particles.
The majority view in the grain-processing industry today is that dry cleaning gives a cleaning effect equal to what could be obtained only with additional washing or a water bath up to a few years ago. Apart from the economy of any particular process, an increasingly important consideration is that the equipment required for the process should produce the minimum environmental pollution. Except in special cases, therefore, wet cleaning, with the large quantities of waste water that it produces, is no longer used.
The main object of this invention is to improve the dry cleaning of grain mainly in grain silos and mills, for preparation for grinding by means of roller mills, and also for cleaning seed grain. Numerous industrial grading processes are known, for example as in the processing of sand and gravel. It is an interesting fact that the production of the highest grades of, for example, concrete gravel, entails the screening and washing thereof. One reason for this is that earthly constituents, and particularly organic constituents, can only be reliably separated with wash water because of their different specific gravity.
Grain cleaning has to deal with separation by size, separation by specific gravity, and in addition separation by external shape, e.g. long grain and round grain. All three separation problems are today solved by purely dry methods in grain cleaning, despite the fact that the mass flows involved are up to 150 t and more per hour per machine, and this means quantities which are not very far away from the dimensions of the corresponding sand-preparing plant.
In operation, the mass flows are deflected and divided in such a way that undesirable impurities are not carried along with the grain.
An existing grain cleaning system comprises the following five main process stages:
1. Coarse pre-screening--a coarse screen frequently combined with a sand screen PA1 2. Separation into heavy and light fractions--concentrator PA1 3. Settling out of stones and the like--stone separator PA1 4. Separation of light impurities--light particle separator and/or aspiration channel PA1 5. Surface cleaning--scourer with a cylindrical screen and rotor.
The main impurities separated are sand and stones, earthy constituents, foreign seeds, husks and straw, large particles such as pieces of wood, and dust so that ultimately the sound grain is obtained as a clean fraction.
The term sand denotes all sandy particles smaller than the size of the sound grain particles. To avoid the sand fraction containing too many grain particles, the smaller grain particles are preferably used as a reference size in practice to determine the perforation of the sand screen. However, it must be remembered that the effective aperture size in a vibrating screen is somewhat smaller than the actual size of the aperture because of the vibration and any inclination.
The degree of separation with the equipment used at the present time in grain cleaning is very high, both for the light constituents and for the heavy constituents. Of course the separation of small stones, sand particles, and so on, is more difficult without the use of water because their specific gravity is only about twice that of the grain particles, and this of course is independent of size. The stones are practically 100% separated in a pulsating air current provided they are larger than the grain particles, but the separation of sand using the stone separator is imperfect. If the sand is finer than the grain particles, it is child's play to screen sand out of a large quantity of larger grain particles, just by screening. This applies on both the small scale and a large scale. The technical solution has long been known, the sand screen being subjected to such loading, or the plant being so enlarged, that practically 100% of the sand fraction can fall through the screen and be separated. However, to increase the screen output requires extra investment, extra energy and extra space.
At the present time, the said sand fraction is not screened out at the beginning as would be expected, but in most cases is done in the following stages. The sand which does not fall through the first screen is concentrated into a light fraction in the concentrator, and then in the next stage is, as it were, rubbed through the surrounding screen of a scourer by means of a high-speed rotor having a large number of percussive strips. Obviously, the larger the sand fraction, the greater the wear on high-speed parts. The fact that a large proportion of sand accumulates at two or three places in the screening circuit is particularly undesirable in many cases, and in particular it makes the setting up of all the machines difficult.
If a high proportion of sand is left in the grain, then the following machines--and particularly the rolls of roller mill trains--will be subjected to correspondingly greater wear. The resulting flour has a relatively high ash value and in many cases this is noticed only occasionally or else only after some period of operation; it is then frequently tolerated.
The main object of the invention is to improve dry cleaning, particularly in respect of sand separation. A decisive factor to this solution is that it should be little or no more expensive in terms of construction and operation than the cleaning plant conventional heretofore.